Compact micro-porous media degasser

ABSTRACT

A degasser ( 10 ) for molten metal with a microporous plate ( 11 ). The microporous has at least one internal passageway ( 13 ) and an interface tube ( 12 ) attached to the microporous plate in flow communication with the internal passageway.

BACKGROUND OF THE INVENTION

The present invention is related to purification of molten metal. Moreparticularly, the present invention is directed to the removal ofhydrogen gas and insoluble impurities from molten aluminum.

Hydrogen is the only gas with significant solubility in molten aluminum.The solubility of hydrogen in molten aluminum is illustrated in FIG. 1.As the temperature of molten metal decreases to the solidificationtemperature the solubility of hydrogen drops significantly. Thissignificant drop results in the formation of undesirable micro-shrinkageand porosity in the final solidification structure. As indicated in FIG.1, about 5% of the hydrogen in the molten aluminum remains aftercompletion of the solidification. The remaining 95% is rejected into theliquid until the concentration reaches the point where a hydrogen gasbubble is formed.

Contact of molten aluminum with ambient water moisture is nearlyunavoidable under reasonable manufacturing conditions. Unfortunately,molten aluminum is highly reactive and can easily reduce, or decompose,any water present by the reaction:H₂O(g)+⅔Al(1)

⅓Al₂O₃₊2H

The removal of hydrogen down to an acceptable level prior tosolidification is required to obtain a metallurgically sound ingot orcasting. The industry accepted practice to remove or lower the dissolvedhydrogen content is to bubble an inert or semi-inert purging gasdirectly through the molten aluminum prior to casting andsolidification. The technology related to purging molten aluminum withinert gas is exemplified in U.S. Pat. No. 5,340,379.

Hydrogen dissolved in molten aluminum exhibits a high vapor pressurerelative to common alloying constituents and impurities. Therefore,hydrogen can be preferentially removed by purging with inert gas or byvacuum treatment. Hydrogen dissolved in molten metal is removed by therecombination of molecular hydrogen to form hydrogen gas based on thefollowing reaction:H=½H₂(gas)

The chemical equilibrium (KH ₂) of the reaction is a function of thepartial pressure (ρ) given by:KH ₂=ρH₂ ^(1/2)

For pure molten aluminum KH ₂ is given by:Ln (K H)=5869/T+3.282

There are several ways to directly introduce purging gas into moltenaluminum to reduce the hydrogen content. A common method includes theuse of a simple pipe or lance, a porous plug, a spinning nozzle degasseror a high-pressure nozzle injection. Exemplary references include U.S.Pat. Nos. 5,340,379; 5,660,614; 6,056,803 and references cited therein.

The rate of removal, and the final hydrogen value obtained, is dependenton several parameters such as the metal temperature, thermodynamicsolubility, purging gas flow rate, metal flow rate in the case ofcontinuous degassing, furnace size in the case of static degassing, gasremoval ratio and bubble size or surface area. For a given purge gasflow rate the hydrogen removal rate is controlled by the bubble size.The finer the bubble size the higher the rate of diffusion and thereforethe higher the rate of removal. A simple lance or tube produces a verylarge bubble size and therefore results in a relatively slow removalrate. The removal rate is improved by introducing the gas through aporous plug or by a spinning rotor that shears the gas stream into finebubbles. The finer bubble size results in increased contact surface areawith an increased transfer rate and slower bubble ascent rate based onthe smaller Stoke's diameter.

There are several limitations in using inert gas bubbles to removehydrogen from molten aluminum. Efficient removal requires the gasbubbles to be relatively small in order to maximize contact surfacearea. The smallest gas bubbles are typically obtained with a rotaryimpeller degasser. The degassers are capable of producing very finebubbles that can remain suspended for a long period of time. As a resultrotary impeller degassers are normally installed a relatively fardistance from the casting machine in order to allow sufficient time forgas bubbles to separate by flotation. This distance also allows ampletime for re-absorption of hydrogen back into the molten aluminum fromatmospheric moisture as well as moisture containing refractory contactmaterials. The lowest achievable hydrogen content is temperaturedependent based on hydrogen solubility-temperature equilibrium. Thelower the temperature at which the hydrogen removal process isconducted, the lower the final hydrogen content at solidification.Ideally, hydrogen removal should be made just prior to the onset ofsolidification, which is not compatible with gas purging.

While rotary impeller degassers are sufficient for generating finebubbles other problems are created by their use. It is known thatfiltration, utilizing either a deep bed or ceramic foam filter, isrequired in addition to degassing. These combined systems typicallyutilize a significant amount of floor space and require that the moltenmetal be held between casts in one, or both, treatment units. Holdingmolten metal creates specific problems. First, an external heat sourcemust be employed to maintain the temperature of the molten metal betweencasts. This requires an elaborate heating system which is a significantcapital expense and has an attendant energy consumption which isexpensive and variable. Secondly, the treatment unit must be drained andrefilled to change the alloy composition. Draining and refilling is asignificant drain on resources requiring non-production labor cost,conversion cost, and productivity losses due to the equipment downtimerequired for the transition. A compact degasser has been described in P.D. Waite, “Improved Metallurgical Understanding Of The Alcan CompactDegasser After Two Years Of Industrial Implementation In AluminumCasting Plates”, Conference Proceedings at the 127^(th) TMS/AIME AnnualMeeting, San Antonio, February 1998, pages 791-796. This system, whilefully drainable, is not compact by current standards. The system alsorequires substantial ancillary support equipment for the launderincluding a degassing hood, baffle plates, drive modules includingrotors, lifting mechanism, fume exhaust system, PLC panel andinterface/gas mixing panel.

A particular problem with the prior art methods of degassing aluminum isthe difficulty associated with monitoring the efficiency of thedegassing operation. It is well known that a system which can not beeffectively monitored can not be optimized for performance.

Summarily, the art has been lacking a suitable degassing and filteringsystem and apparatus.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedapparatus, and method, for degassing molten metal, preferably aluminum.

It is another object of the present invention to provide an apparatus,and method, for degassing molten metal, preferably aluminum, which isefficient and which requires a lower investment with regards toequipment and space than previous methods.

It is another object of the present invention to provide an apparatus,and method, for degassing aluminum whereby the efficiency of thedegassing operation can be monitored for efficiency and optimized forperformance.

A particular feature of the present invention is the ability toincorporate the invention in existing environments with minimalalterations.

Another particular feature of the present invention is the ability toincorporate the invention into new installations thereby greatlyenhancing the efficiency of the casting operation.

These and other advantages, as will be realized from the descriptionherein, are provided in a degasser, 10, for molten metal with amicroporous plate, 11. The microporous plate has at least one internalpassageway, 13, and an interface tube, 12, attached to the microporousplate in flow communication with the internal passageway.

Yet another embodiment is provided in a method for purifying moltenmetal. The method includes melting metal to form molten metal. Themolten metal is passed through a containment vessel wherein thecontainment vessel has a degasser and the degasser has a microporousplate with at least one internal passageway and an interface tubeattached to the microporous plate and in flow communication with theinternal passageway. Hydrogen is removed from the microporous platethrough the interface tube.

A particularly preferred embodiment is provided in apparatus forpurifying molten metal. The apparatus has a containment vessel with aninlet throat and an outlet throat. A degasser is between the inletthroat and the outlet throat. The degasser has a microporous plate withat least one internal passageway and an interface tube attached to themicroporous plate in flow communication with the internal passageway.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart depicting solubility of hydrogen in aluminum.

FIG. 2 is a cross-sectional view of a degasser plate of the presentinvention.

FIG. 3 is a front cross-sectional view of a degasser plate of thepresent invention.

FIG. 4 is a cross-sectional side view of the degasser plate of FIG. 3.

FIG. 5 is a perspective schematic view of a degasser plate of thepresent invention as visualized during use.

FIG. 6 is a schematic view of an embodiment of the present invention asemployed in a casting launder.

FIG. 7 is a top view of an embodiment of the present invention asemployed in a filter bowl.

FIG. 8 is a cross-sectional schematic view of an embodiment of thepresent invention.

FIG. 9 is a cross-sectional view of an embodiment of the presentinvention.

FIG. 10 is a cross-sectional view of an embodiment of the presentinvention being heated prior to use.

FIG. 11 is a perspective view of a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is specific to an apparatus, and method, fordegassing aluminum which is compact, efficient, and which can bemonitored for optimization. In general, the present invention utilizes amicro-porous plate, or panel, which is immersed in the molten metal andwhich removes hydrogen by diffusion into the plate for removal by purgeor vacuum. The invention will be described with reference to the variousdrawings which form an integral part of the present invention. Thedrawings are illustrative and not intended to limit the invention. Inthe various drawings similar elements will be numbered accordingly.

The porous media degasser comprises a micro-porous plate, or panel, thatis submerged into the molten metal such as aluminum, steel, copper oriron, to be degassed. An embodiment of the present invention isillustrated in FIG. 2, wherein the degasser, generally represented at10, is shown in cross-sectional view. The degasser, 10, comprises amicro-porous plate, 11, with at least one interface tube, 12, interfacedto internal passageways, 13. The upper extent of the plate, 11,comprises a seal, 14. The interface tube, 12, removes hydrogen gas byvacuum or by purging. In a vacuum arrangement the interface tube, 12, isattached to a vacuum which causes a decreased pressure in the plate. Anygases contained therein are removed. With a vacuum arrangement a singleinterface tube, and passageway, can be employed but multiple interfacetubes and passageways are preferred. In a purge arrangement anon-reactive purging gas, preferably argon or nitrogen, is introduced toone interface tube and exhausted from the other interface tube. Theexhaust can be with vacuum assist if desired.

The interface tubes are non-porous, preferably metal or dense ceramicsuch as graphite, boron nitride, alumina, zirconia or mullite.Preferably, the interface tubes could be constructed of steel oraustenitic stainless steel. The tubes can be coated, to preventdissolution, with a material such as plasma coated alumina or zirconiaor the tubes could be coated with a material such as boron nitride.

While not limited to any theory, the function of the interface tubes andassociated internal passageways is to provide a continuous flow ofpurging gas. The flowing purging gas continuously removes hydrogen gaswhich is formed by hydrogen atoms diffusing into the porous plate andreacting therein to form hydrogen gas. By continuously removing thehydrogen gas a high driving force is maintained for the diffusion ofhydrogen atoms, or cations, into the plate. Either a purge or a vacuumremoves the hydrogen by the same basic mechanism based on the partialpressure of hydrogen in the plate relative to the molten metal.

The presence of hydrogen in argon has a significant impact on thethermal conductivity. This change in thermal conductivity can bemeasured and quantified using commercial thermal conductivity analyzers.By measuring the purging gas flow rate and the % hydrogen gas in theargon, based on the conductivity, the performance of the degasser can bemeasured in real-time and the performance optimized with regards to flowrates and volumes of purge air. Due to the enhanced ability to monitorefficiency a purge system is preferred over a vacuum system.

Because the purge gas pressure is lower in the micro-porous plate thanthe surrounding metallostatic pressure, the purge gas is retained in themicro-porous plate. The micro-porous plate is structurally designed suchthat the micro-porous material is not penetrated by the molten metal butis permeable with respect to the hydrogen cation in the molten metal.

The micro-porous plate microstructure and material are selected suchthat capillary penetration of the molten metal into the micro-porousmaterial will not occur. Material factors that control capillarypenetration are the molten surface energy (γ_(is)), the metal-materialwetting angle (θ), and the metallostatic head pressure (H_(p)). Thecritical metallostatic pressure (H_(p)) required to penetrate amicro-porous material is defined as:H _(p≧)4γ_(is)(cos θ)/gρφ

wherein, Hp is the critical pressure for capillary penetration, γ_(is)is the interfacial surface energy between the porous media and themolten aluminum, θ is the contact wetting angle of molten aluminum onthe porous media, g is Newton's constant, ρ is the liquid metal densityand φ is the pore opening size of the porous metal. Therefore, thecalculated H_(p) must be substantially higher than the actual capillarypressure at a given immersion depth.

By selecting a micro-porous material with appropriate γ_(is) and θvalues for a given molten metal immersion depth to maintain asufficiently high H_(p), the micro-porous plate will resist capillarypenetration of the molten metal yet will remain permeable to both thehydrogen gas and the purge gas required to remove the hydrogen gas.

A wide range of porous material would be suitable for demonstration ofthe present invention within the context of permeability, as set forthpreviously, and the understood desire to have a non-reactive material.

Particularly preferred materials include rigidized vacuum formed fiberboards, open cell reticulated ceramic foam, ceramic foam withmicro-porous coating, bonded particle materials and ceramic materialswhere an organic pore former material, such as walnut flour, organicmicrospheres, saw dust or the like is added to the slurry and is burnedout during firing.

Rigidized vacuum formed fiber boards are materials based on aluminumsilicate, silica, magnesium silicate or alumina fibers typically bondedwith either colloidal silica or alumina. The fiber microstructure isextremely fine and open with 60-70% open pore volume. These materialshave excellent thermal shock resistance due to their discontinuous fibermatrix. Vacuum formed fiber boards have low thermal diffusivity andtherefore do not chill, or freeze, the molten aluminum on initialcontact. Commercial rigidized vacuum formed boards are availablecommercially form either Zircar Ceramics Inc. of Florida, N.Y. or RathPerformance Fibers of Wilmington Del.

Open cell reticulated ceramic foams are completely open cell with adiscontinuous structure. To prevent metal penetration a relatively finepore size, preferably greater than 60 pores per inch, would be necessaryunless coatings were incorporated to form a micro-porous coating.

An alternative embodiment is illustrated in front cross-sectional viewin FIG. 3. A side cross-sectional view is provided in FIG. 4. Thedegasser of FIGS. 3 and 4, comprises a hollow, micro-porous plate, 20,with at least one interface tube, 12, in flow communication with acavity, 21, interior to the plate. A particular advantage of theembodiment of FIGS. 3 and 4 is the elimination in pressure drop causedby the purge gas being forced to migrate through the porous material.

It is preferred that the micro-porous plate be submerged entirely belowthe surface of the molten metal to avoid creation of a flow path toambient atmosphere. A top perspective schematic view of a degasser ofthe present invention as employed is provided in FIG. 5. In FIG. 5, thedegasser, 10, comprising a pair of interface tubes, 12 and 12′, issubmerged in molten metal, 24, in a crucible, 25. Purge gas is providedby a source, 26, to the interface tube, 12, and exhausted from interfacetube, 12′, with hydrogen gas included therein. An auxiliary unit, 27,such as a vacuum pump or thermal conductivity analyzer is in flowcommunication with the exhaust interface tube, 12′.

In a continuous casting process the micro-porous plates could beinstalled in a wide range of locations depending on the specifics of thecasting operation. In the case of billet or ingot casting themicro-porous plates could be installed in a casting launder asillustrated in FIG. 6 wherein the launder may be before or after thefilter bowl. In the embodiment illustrated in FIG. 6, a multiplicity ofdegassers, 10, each with an inlet interface tube, 12, and exhaustinterface tube, 12′ are employed in a casting launder, 90. Themultiplicity of inlet interface tubes, 12, are in flow communicationwith a gas manifold, 30, for supply of non-reactive gas to thedegassers. Similarly, the multiplicity of exhaust interface tubes, 12′,are in flow communication with an exhaust manifold, 30′. It would beunderstood that each degasser may have a unique gas supply and exhaustand that different degassers may have different arrangements. Forexample, in a multiple degasser arrangement, some degassers may employ apurge mechanism while others may employ a vacuum mechanism.

The degasser may be employed in the filter bowl as illustrated in FIG.7, wherein the degasser, 10, and interface tubes, 12, are as describedpreviously, and the filter bowl is indicated at 33. The filter, 34, ispreferably a porous ceramic filter.

In continuous strip casting the porous plates could be installed in thecasting launders, filter bowl, head box or embedded within the castingtip.

The degasser may be integral to the launder as illustrated in FIG. 8. InFIG. 8, the interface tubes, 12, are in flow communication with acavity, 40. The interior walls, 41, are porous as set forth previously.The exterior walls, 42, are preferably non-porous or, alternatively, theexterior shell, 43, prohibits purge gas from exiting the localizedenvironment of the launder.

A preferred embodiment of the present invention will be described withreference to FIG. 9. In FIG. 9, the micro-porous plate degasser,generally represented at 50, comprises a refractory containment vessel,51, comprising an inlet throat, 52, and outlet throat, 53. The inletthroat receives molten metal and directs it through the vessel towardsthe outlet throat and to a reconnecting launder, or subsequent device,which is not shown.

Between the inlet throat and outlet throat is a degasser plate, 54, andoptional filter, 55. The molten metal preferably passes through thedegasser plate for degassing, as described previously, followed byflowing through the filter wherein insoluble materials are removed. Inother embodiments the degasser can be downstream of the filter and inretrofit applications this may be preferred due to immutable restraintssuch as space, cost, overall layout etc. The filter can be removed, ornot incorporated, when filtration is not required or is accomplishedseparately.

The degasser, 54, comprises a degasser plate, 56, and an associatedinterface tube, 57, located in a recess, 58. The recess is preferablytapered, wherein a substantial portion of the molten metal must gothrough, not around, the degasser plate, 56. A vacuum is drawn throughthe interface tube, 57, as described previously. Alternatively, a secondinterface tube, and purge gas, may be employed as previously described.

The filter is preferably separated from the degasser, 54, by anequalization space, 60, to allow the degassed molten metal to spreadover the surface of the filter element to improve filtration efficiency.The equalization space is preferably at least about 6 mm to about 55 mm.Below about 6 mm the separation is insufficient to insure adequatespread of the molten metal over the surface of the filter. Above about55 mm the advantages diminish resulting in an increased size of theentire system which is not desirable.

After passing through the degasser plate and filter, in either order,the molten metal enters a first transition region, 61, comprising adownward sloped floor, 62, and a drain plug, 63, at the lowest extent ofthe floor. The drain plug, 63, can be removed to drain the entireapparatus. After the first transition region the molten metal enters asecond transition section, 64, which connects the degasser assembly to adownstream apparatus such as a transition launder, mold, transportassembly or the like. The second transition section is preferablyoriented such that any non-forced flow would be towards the drain plugto facilitate draining of the degasser assembly. The first transitionsection and second transition section, taken together, represent theoutlet throat, 53. It is preferred that the degasser assembly be drainedbetween casts. When molten metal is not drained between casts it wouldbe realized by those of skill in the art that an external heater isdesired to maintain the stagnant metal in a molten state.

The orientation of the degasser plate is preferably horizontal, relativeto ground, with molten metal flowing down through the plate. Thisorientation insures that the molten metal flows over the entire surfaceand therefore maximizes efficiency. The degasser assembly, and plate,may be in any orientation and molten metal may flow upward in a forcedflow orientation if desired.

The filter may comprise multiple filter plates with the multiple plateshaving the same or different porosity. When multiple filter plates areemployed it is preferred that they be separated to allow the moltenmetal to spread evenly over the face of the second filter. Multi-plateconfigurations are described in U.S. Pat. No. 5,673,902.

Prior to passing molten metal through the degasser assembly it ispreferable to preheat the filter, degasser and containment vessel toprohibit localize solidification of molten metal as it contacts a coolersurface. A preheater is preferably inserted into the outlet throat, asillustrated in FIG. 10. In FIG. 10, the heater, 70, is inserted into theoutlet throat, 53, and heat is directed in a counterflow direction,relative to metal flow, to heat the interior walls of the containmentvessel as well as the filter and degasser.

A particular preferred degasser plate is illustrated in FIG. 11. In FIG.11, the degasser plate, 80, comprises a multiplicity of passages, 81,through the plate. While not limited to any theory, the passages allowmolten metal to pass through thereby reducing the diffusion path lengthfor the dissolved hydrogen to reach the removal interface and increasingthe contact surface area for hydrogen removal.

The passages have an equivalent diameter of at least about 500 micronsto no larger than about 50 mm. More preferably, the passages have anequivalent diameter of 1 to 10 mm and more preferably about 5 to about7.5 mm, Equivalent diameter is the diameter of a circle with the samecross-sectional area as the passage. Round passages are preferred due tomanufacturing convenience

The spacing between passages, measured from the center of each passage,is preferably about ½ to 10 times the hole diameter. Optimum holespacing is about 3 mm to about 10 mm. The pattern of passages ispreferably either simple orthogonal or close packed array with closepacked array being preferred.

The degasser plate thickness can range from about 3 mm to about 200 mm.If passages are employed the plate can be thicker than if passages arenot employed. A plate thickness of about 25-100 mm is most preferred forstandard operations.

The preheater is preferably a medium velocity burner with excess aircapability. Burners using above 100%, excess air, are preferred and thedegasser assembly and filter assembly are heated by convective heattransfer.

The present invention has been described with particular reference tothe preferred embodiments which are intended to illustrative but are notto be considered limiting. Other configurations, alterations andembodiments could be realized from the teachings herein withoutdeparting from the scope of the invention which is set forth moreclearly in the claims appended hereto.

1. A degasser for molten metal comprising: a microporous platecomprising at least one internal passageway and an interface tubecapable of removing gas from said internal passageway attached to saidmicroporous plate in flow communication with said internal passagewayand further comprising a vacuum pump attached to said interface tube. 2.The degasser of claim 1 further comprising a second interface tube inflow communication with a second internal passageway.
 3. The degasser ofclaim 2 wherein said second internal passageway and said internalpassageway form a cavity.
 4. The degasser of claim 1 wherein saidpassageway comprises passages with an equivalent diameter of at leastabout 500 microns to no larger than about 50 mm.
 5. A degasser formolten metal comprising: a microporous plate comprising at least oneinternal passageway and an interface tube attached to said microporousplate in flow communication with said internal passageway wherein saidpassageway comprises passages wherein said passages have an equivalentdiameter of at least about 5 mm to no more than about 7.5 mm.
 6. Adegasser for molten metal comprising: a microporous plate comprising atleast one internal passageway and an interface tube attached to saidmicroporous plate in flow communication with said internal passagewaywherein said microporous plate comprises passages and wherein saidpassages are separated by a distance between about 0.5 to 10 times anequivalent diameter of said passage.
 7. The degasser of claim 1 whereinsaid microporous plate is about 3 mm to about 200 mm thick.
 8. Thedegasser of claim 1 further comprising a containment vessel with saidmicroporous plate contained in said containment vessel.
 9. The degasserof claim 8 further comprising a filter in said containment vessel. 10.The degasser of claim 1 further comprising a monitor in flowcommunication with said interface tube for monitoring gases flowingtherethrough.
 11. The degasser of claim 1 further comprising a purgearrangement attached to said interface tube.
 12. The degasser of claim 2further comprising a vacuum pump attached to said second interface tube.13. The degasser of claim 2 further comprising a purge arrangementattached to said second interface tube.
 14. The degasser of claim 5further comprising a second interface tube in flow communication with asecond internal passageway.
 15. The degasser of claim 5 wherein saidsecond internal passageway and said internal passageway form a cavity.16. The degasser of claim 5 wherein said microporous plate is about 3 mmto about 200 mm thick.
 17. The degasser of claim 5 further comprising acontainment vessel with said microporous plate contained in saidcontainment vessel.
 18. The degasser of claim 17 further comprising afilter in said containment vessel.
 19. The degasser of claim 5 furthercomprising a monitor in flow communication with said interface tube formonitoring gases flowing therethrough.
 20. The degasser of claim 6further comprising a second interface tube in flow communication with asecond internal passageway.
 21. The degasser of claim 6 wherein saidsecond internal passageway and said internal passageway form a cavity.22. The degasser of claim 6 wherein said passages have an equivalentdiameter of at least about 500 microns to no larger than about 50 mm.23. The degasser of claim 6 wherein said microporous plate is about 3 mmto about 200 mm thick.
 24. The degasser of claim 6 further comprising acontainment vessel with said microporous plate contained in saidcontainment vessel.
 25. The degasser of claim 24 further comprising afilter in said containment vessel.
 26. The degasser of claim 6 furthercomprising a monitor in flow communication with said interface tube formonitoring gases flowing therethrough.
 27. The degasser of claim 5further comprising at least one of a vacuum pump attached or a purgearrangement attached to said interface tube.
 28. The degasser of claim 6further comprising at least one of a vacuum pump attached or a purgearrangement attached to said interface tube.